scsi: scsi_debug: Fix sdebug_blk_mq_poll() in_use_bm bitmap use

The in_use_bm bitmap of struct sdebug_queue should be accessed under
protection of the qc_lock spinlock. Make sure that this lock is taken
before calling find_first_bit() at the beginning of the function
sdebug_blk_mq_poll().

Link: https://lore.kernel.org/r/20220404045547.579887-1-damien.lemoal@opensource.wdc.com
Fixes: 3fd07aec ("scsi: scsi_debug: Fix qc_lock use in sdebug_blk_mq_poll()")
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_debug.c

@@ -7519,12 +7519,13 @@ static int sdebug_blk_mq_poll(struct Scsi_Host *shost, unsigned int queue_num)
 	struct sdebug_defer *sd_dp;
 
 	sqp = sdebug_q_arr + queue_num;
-	qc_idx = find_first_bit(sqp->in_use_bm, sdebug_max_queue);
-	if (qc_idx >= sdebug_max_queue)
-		return 0;
 
 	spin_lock_irqsave(&sqp->qc_lock, iflags);
 
+	qc_idx = find_first_bit(sqp->in_use_bm, sdebug_max_queue);
+	if (qc_idx >= sdebug_max_queue)
+		goto unlock;
+
 	for (first = true; first || qc_idx + 1 < sdebug_max_queue; )   {
 		if (first) {
 			first = false;
@@ -7589,6 +7590,7 @@ static int sdebug_blk_mq_poll(struct Scsi_Host *shost, unsigned int queue_num)
 			break;
 	}
 
+unlock:
 	spin_unlock_irqrestore(&sqp->qc_lock, iflags);
 
 	if (num_entries > 0)